Zachary A. Roth

Guided Mode Resonance Filter as a Spectrally Selective Feedback Element in a Double-Cladding Optical Fiber Laser

In this work, a spectrally selective optical element is introduced based on a 2-D guided mode resonance filter (GMRF) as an external feedback element. The GMRF was designed to provide a highly efficient narrow linewidth reflection within the gain bandwidth of the fiber laser, while transmitting the pump beam. These features enabled the fiber laser to operate in an external cavity configuration to provide a wavelength-stabilized and narrow linewidth output within the optical -band.

Nanofabrication of a space-variant optical transmission filter

A space-variant optical transmission filter is demonstrated for which a simplified process is used to tailor the spatial response of the filter across the surface of a single wafer. A multilayer stack, of alternating high or low refractive index dielectric materials, was used to produce a narrow transmission notch in the center of a wide stop band. Subsequent patterning and etching of arrays of holes through the volume of the dielectric stack was performed to control the fill factor of the dielectric in the layers. The position of the transmission notch within the reflection spectrum was varied across the device surface by adjusting the hole diameter of the hole arrays. Experimental and numerical simulation were used to confirm the space-variant transmission characteristics of a single-wafer sample with two zones of different hole diameter arrays in the 1550 nm wavelength regime.

Spatial and spectral beam shaping with space-variant guided mode resonance filters

Novel all-dielectric beam shaping elements were developed based on guided mode resonance (GMR) filters. This was achieved by spatially varying the duty cycle of a hexagonal-cell GMR filter, to locally detune from the resonant condition, which resulted in modified wavelength dependent reflection and transmission profiles, across the device aperture. This paper presents the design, fabrication, and characterization of the device and compares simulations to experimental results.

Novel method for the fabrication of spatially variant structures

Spatially varying grating structures formed at the subwavelength scale behave as a layer with an artificial effective refractive index that is dependent on the local fill fraction. We describe a novel technique to pattern gratings with a spatially varying fill fraction using a simple two-step exposure process. The first exposure forms a partial latent image of a grating in the photoresist. The resist is then saturated by overlaying an exposure with an analog spatially varying intensity, generated by using a phase-only masking technique. The cumulative exposure dose from the two steps was designed so that the point of minimum intensity will still develop the photoresist through, in all the spaces in the grating. By varying the exposure window around the saturation dose, the fill fraction of the patterned gratings was modulated; thus, the size of the space cleared at any location in the grating is a scalable function of the local cumulative dose delivered. Constant feature height is achieved across the patterned area by keeping the second exposure dose below the resist threshold exposure value. The exposure process was modeled numerically to predict the relationship between the local dose and patterned fill fraction. This technique enables rapid, low-cost fabrication of apodized grating structures for applications in diffractive optics technology.

Two-dimensional guided mode resonance filters fabricated in a uniform low-index material system

We demonstrate the fabrication, simulation, and experimental results of a buried, homogeneous narrowband spectral filter with a periodic, hexagonal unit cell of air pockets, encapsulated in a fused silica substrate. The leaky waveguide is formed by depositing SiO(x) on an etched fused silica grating via plasma-enhanced chemical vapor deposition. Design principles of guided mode resonance filters were utilized to achieve a resonance with 60% reflectivity at a wavelength of 1.741 μm. The device demonstrates resonance with FWHM of 6 nm.

Polarization selective, graded-reflectivity resonance filter, using a space-varying guided-mode resonance structure.

We designed, fabricated, and tested, polarization selective, graded-reflectivity resonant filters; based on a radial-gradient spatially-distributed, guided-mode resonance device architecture. The demonstrated filters have polarized spectral-resonance responses, distributed across their aperture extent, in the range between 1535 nm and 1540 nm wavelengths. Spectral sensitivity was observed on device tests, for wavelength changes as low as 0.2 nm. Using multiple lithographic exposures and biasing exposure methods, the devices were engineered to have a sub-aperture region, with no hard boundaries or diffraction anomalies.

Biologically inspired optics: analog semiconductor model of the beetle exoskeleton

Evolution in nature has produced through adaptation a wide variety of distinctive optical structures in many life forms. For example, pigment differs greatly from the observed color of most beetles because their exoskeletons contain multilayer coatings. The green beetle is disguised in a surrounding leaf by having a comparable reflection spectrum as the leaves. The Manuka and June beetle have a concave structure where light incident at any angle on the concave structures produce matching reflection spectra. In this work, semiconductor processing methods were used to duplicate the structure of the beetle exoskeleton. This was achieved by combining analog lithography with a multilayer deposition process. The artificial exoskeleton, 3D concave multilayer structure, demonstrates a wide field of view with a unique spectral response. Studying and replicating these biologically inspired nanostructures may lead to new knowledge for fabrication and design of new and novel nano-photonic devices, as well as provide valuable insight to how such phenomenon is exploited.

Azimuthally Varying Guided Mode Resonance Filters

New and novel sensing schemes require optical functions with unconventional spatial light distributions, as well as complex spectral functionality. Micro-optical elements have shown some flexibility in their ability to spatially encode phase information using surface relief dielectrics. In this paper, we present a novel optical component that exploits the properties of optically resonant structures to make an azimuthally spatially varying spectral filter. The dispersive properties are quite unique with an angular resonance shift of 28 Deg/nm. This device is fabricated using techniques that are compatible with standard micro-electronic fabrication technologies.

Advanced fabrication methods for 3D meta-optics

Micro-Optics has expanded to include a wide variety of applications for spectral filtering, polarization filtering and beam shaping. Recently, a new class of optical elements have been introduced that can combine the spectral, polarization, and beam conditioning into the same optical element. This engineered optical functionality results in a 3D Meta-Optic structure that relies on sub-wavelength features to essentially engineer the electromagnetic fields within the structure; thereby, resulting in highly dispersive structures that spatially vary across the optical element. This talk will summarize recent results in the design, fabrication and applications of 3D Meta-Optics.

Narrow Linewidth, Tunable, CW, Thulium Fiber Lasers with VBG and GMRF stabilization

Eye-safe, high power tunable narrow linewidth lasers are important for various applications such as atmospheric propagation measurements. We have investigated two techniques of generating narrow linewidth thulium 2-μm fiber lasers, utilizing a reflective volume Bragg grating (VBG), and a guided mode resonance filter (GMRF) as a cavity end mirror. A stable narrow linewidth (50 pm), tunable (from 2004 nm to 2054 nm) thulium doped fiber laser using a reflective VBGg was demonstrated. A CW power of 17 W was achieved. Using a GMRF as an end mirror we showed a narrow linewidth (~30 pm) laser with an output power of 5.8W, and at a slope efficiency of 44%.